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Towards a Comparative Study of Animal Consciousness
Walter Veit
The University of Sydney, Camperdown, NSW, Australia
University of Cambridge, Cambridge, UK
Abstract
In order to develop a true biological science of consciousness, we have to remove
humans from the center of reference and develop a bottom-up comparative study of
animal minds, as Donald Griffin intended with his call for a “cognitive ethology.” In
this article, I make use of the pathological complexity thesis (Veit 2022a, b, c) to show
that we can firmly ground a comparative study of animal consciousness by drawing
on the resources of state-based behavioral life history theory. By comparing the
different life histories of gastropods and arthropods, we will be able to make better
sense of the possible origins of consciousness and its function for organisms in their
natural environments.
Keywords
Animal consciousness; Arthropods; Comparative cognition; Evolution of consciousness;
Gastropods; Insects; Sentience
Most of Darwin’s basic ideas about evolution are now generally accepted by scientists,
but the notion that there has been evolutionary continuity with respect to conscious
experiences is still strongly resisted. Overcoming this resistance may be the final,
crowning chapter of the Darwinian revolution.
Donald Redfield Griffin (1998, p. 14)
Prelude
This article is a philosophical contribution to the science of animal consciousness-a
science that the prominent American ethologist and discoverer of bat echolocation
Donald Griffin tried to establish in the 1970s when he called for a “cognitive
ethology,” but which only truly began to take shape as a genuine interdisciplinary
field a decade after his death with the “Cambridge Declaration on Consciousness” in
2012 and the formation of the first interdisciplinary journal of nonhuman
consciousness in 2015, aptly titled “Animal Sentience”
1
(Browning and Birch 2022;
1
See Veit and Harnad (2020) and Veit and Rowan (2020) for interviews with its editors.
Please cite as: Veit, W. (forthcoming). Towards a Comparative Study of
Animal Consciousness. Biological Theory.
Check www.walterveit.com for citation details once published
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Veit and Browning 2022). As per the epigraph featured above, this article will try to
help the realization of Griffin’s vision of the “final, crowning chapter of the
Darwinian revolution” by helping this burgeoning field to cast off the chains of a
pre-Darwinian view of the mind in both philosophy and science and begin a
transition towards a true Darwinian science of consciousness in which its
evolutionary origin, function, and phylogenetic diversity are moved from the field’s
periphery to its very center.
In order to develop a true biological science of consciousness, we must attend
to the (cognitive) ethologist’s demand to address the functionalist question of what
consciousness in all of its diversity and gradations does for healthy agents in the
pursuit of their life history strategies. Accordingly, an evolutionary approach to
consciousness has two objectives: (1) to demonstrate the possibility of a comparative
bottom-up approach that addresses the problem of consciousness in terms of the
evolutionary origins of a new life history strategy that made consciousness worth
having, and (2) to articulate a thesis and beginnings of a theory of the place of
consciousness as a complex evolved phenomenon in nature. The thesis that I have
developed for such an evolutionary approach to consciousness is what I have dubbed
“the pathological complexity thesis” (Veit 2022a, b, c). It can be succinctly
summarized as follows:
The Pathological Complexity Thesis:
The function of consciousness is to enable the agent to respond to pathological
complexity.
Pathological complexity can simply be understood as the computational complexity
of the economic trade-off problem between competing actions faced by all organisms
as they deal with challenges and opportunities throughout their life cycle in order to
maximize their fitness. This ecological notion of biological complexity is inherently
evaluative and will vary according to the different life histories of different animals,
dynamically emerging from the interaction of organism and its environment (see also
Veit and Browning forthcoming).
To provide a one-paragraph summary of the pathological complexity thesis:
my framework is intended as a rejection of the false dichotomy between the two
dominant traditions in philosophy of mind and the science of human consciousness,
between strongly externalist representationalist theories of consciousness that
overemphasize sensory experience and strongly internalist ones that overemphasize
self-awareness as the models for all of experience. Instead, I aimed to develop an
alternative model of consciousness based on a model of evaluative experience, which can
be described as an inherently “interactionist” or perhaps better a “dynamic”
dimension of consciousness.
In both my dissertation (Veit 2022c) and a compendium article to this article
(Veit 2022a), I have offered a defense of why the Cambrian explosion led to the dawn
of a new sentient life history strategy. This new mode of being led to individuals
capable of feeling positive and negative experience (hedonic valence)—a capacity
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evolved for efficient action selection that quickly became more representational due
to increases in discriminatory capacities, thus ultimately giving rise to sensory
experience and “points of view” as evolutionary transformations of more basic
Benthamite creatures only capable of evaluative experience.
2
While the original sentient beings plausibly had simple hedonic evaluations
(whether positive or negative) of their current situation that compelled them towards
particular actions, this capacity would have transformed “quickly” over the next
millions of years. Evaluations of particular states would come to be associated with
their own phenomenological character to enable the comparative evaluation of
tradeoffs and associative learning. If this account for the core and origins of
consciousness is on the right track, we will be able to make predictions about the
phenomenological complexity of other animals through an analysis of the
pathological complexity (or life history complexity) of their species-specific lives, thus
enabling us to develop a comparative bottom-up study of consciousness just as
Donald Griffin intended with his call for a cognitive ethology.
The goal of this article will be to put the pathological complexity thesis to the
test, both as a framework for a bottom-up comparative study of animal
consciousness, and as a hypothesis about the core and origins of consciousness being
found in hedonic valence. I will do so by responding to a suggestion by Godfrey-
Smith (2020a, b) that there could be a phylogenetic split between the extant conscious
animals, with some animals having evaluative experience while lacking the sensory
experience and vice versa. Could there be a dissociation between these dimensions
such that some animals only have sensory experience?
If we find animals for which this is the case, that would at least raise an
interesting challenge to the pathological complexity thesis, since my theory locates
minimal consciousness in evaluation and treats sensory experience as an evolutionary
“add-on,” once the basic evaluative capacities gain discriminatory and
representational richness (Veit 2022c). Sensory experience is simply an outcome of
an increase in evaluative complexity that allows for more stimuli to be distinguished,
assigned value, and compared to enable efficient action selection. This account
provides us with an explanation for why some sensory states have a subjective “feel”
to them, making hedonic evaluation a prerequisite for sensory experience.
Information-processing complexity in the sensory processes of an organism alone is
not sufficient to give rise to minimal consciousness in the sense of qualitative
experience, which is why Godfrey-Smith’s arguments for a phylogenetic split
constitute an interesting challenge.
To investigate this potential counter to the pathological complexity thesis, we
will firstly look at the gastropods (snails and slugs) and secondly the arthropods (in
particular crustaceans and insects) that are used in Godfrey-Smith’s case for a
phylogenetic split. Admittedly, only insects constitute a real counterexample because
2
I call these Benthamite after the father of utilitarianism Jeremy Bentham, who argued that
animals and humans act to maximize their hedonic wellbeing. The evolution of these
creatures will also be the subject of a future book on animal consciousness (Veit 2022d).
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Godfrey-Smith (2020b) uses gastropods as a potential case where the sensory side
appears to be very simple but “may have relevant evaluative complexity” (p. 1153).
Indeed, my discussion of gastropods will primarily serve as evidence for the
pathological complexity thesis: the possibility of minimal consciousness in the sense
of hedonic valence, without other dimensions of consciousness being present. This
is entirely compatible with the pathological complexity thesis, and in fact lends
support to it, since it would increase our credence for thinking that evaluative
experience could exist without the sensory side of things. In insects, however,
Godfrey-Smith maintains that they only have simple evaluative capacities, whereas
their sensory capacities are sufficiently rich to make it at least plausible that they could
have sensory experience without the evaluative side. Reviewing the evidence of the
literature for this view, I will ultimately reject it, arguing that insects have sufficient
evaluative complexity to undermine the view that they could have consciousness
without the capacity to have positive or negative feelings. Admittedly, little attention
has so far been given to the gastropods and arthropods in discussions of animal
consciousness, but we can use the pathological complexity framework alongside
recent evidence to move us further towards understanding what it is like to be them.
Article Outline
This article is structured as follows. In the second section, “Gastropods: A Sluggish
Way of Life,” I use the case of gastropods to support the motivation of the
pathological complexity thesis in seeking the origins of consciousness in evaluation.
Indeed, they serve as plausible model organisms to study the origins of sentience,
with the other dimensions idealized away by nature itself. In the third section,
“Arthropods: A Robotic Way of Life,” I respond to the challenge that insects might
have sensory experience without evaluative experience and argue that even land
insects share many similarities with the life histories of crustaceans, which should
provide evidence against the idea that insects do not have evaluative experience, even
if they do not feel pain. Pain may be the human paradigm for hedonic valence, but
thirst, hunger, and other evaluative processes such as learning, alongside other long-
term states, may be present even in its absence. Finally, the fourth section,
“Conclusion and Further Directions,” will summarize the arguments of this article,
offer some responses to potential objections, and explore potential directions for the
further development of the pathological complexity framework for a comparative
study of animal consciousness.
Gastropods: A Sluggish Way of Life
The first class of animals we shall discuss are gastropods (i.e., snails and slugs) which,
like cephalopods, are molluscs, though they generally differ in nervous system
complexity. Unlike cephalopods, and in particular octopuses (Mather 2008; Browning
2019a; Jacquet et al. 2020; Schnell et al. 2022), gastropods have received little
attention in debates on animal consciousness. Their lives appear too slow, too
uninteresting, compared to the extreme behavioral flexibility, tempo, and intelligence
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of their octopus relatives. One might thus be tempted to categorize their pathological
complexity as nonsignificant, but that would be a mistake. As Dennett (2019) once
warned, our imagination is in many ways shaped by what Wittgenstein dubbed
Lebensform (form of life), that is, “our linguistic communities, the commonalities that
are apt to confound our thinking with parochiality” (p. 2). If we observe animals only
distantly related to us and with very different ways of life, we will be influenced by
what Dennett nicely expressed as their behavioral rhythm and speed:
[I]f cephalopods moved in the clunky way of most existing robots, then in spite
of the manifest purposiveness of their motions, it would be quite comfortable
to suppose that they were some kind of zombies, marine robots with eight or
ten appendages. (Dennett 2019, p. 2)
Gastropods, of course, appear even slower than many sophisticated robots. Care
must be taken not to deny them consciousness simply because they are different.
Prominently, Feinberg and Mallatt (2016) argue that evidence for consciousness in
gastropods is lacking, but they also admit that there is some evidence pointing
towards the affective side. However, they end up denying consciousness to
gastropods since they are said to “lack the brain complexity one would expect for
consciousness” (2016, p. 192). This, of course, raises the question of whether we
already know what brain complexity would be necessary. Godfrey-Smith (2020b)
evaluates the evidence in a different way by emphasizing that gastropods may be a
case with sufficient richness in evaluative capacities to have evaluative consciousness
while lacking the other dimensions. If so, this would provide strong support for the
pathological complexity thesis: we could have animals around us in the here and now,
rather than just at the origin of consciousness in the Cambrian, with a minimal sense
of hedonic evaluation without the other dimensions. A theory of consciousness based
on the human case is undoubtedly prone to fail in its recognition of such “marginal”
cases, so it is useful to examine their life histories from their own point of view by
using the pathological complexity framework.
Evaluative Experience
In his emphasis on the evaluative capacities of gastropods, Godfrey-Smith draws
especially on the work of Terry Walters, who has been one of the frontrunners in
advancing our understanding of gastropod skills.
3
Notably, we should not simplify
this dimension to only pleasure and pain. These are often used in a very deflationary
sense for any sort of subjective experience that has a positive or negative valence (see
also Browning 2020). For obvious reasons this can mislead others to needlessly
restrict this dimension, missing out on medium-term and long-term states such as
emotions of anger or fear. As we will also see in the following discussions of insects,
we should be open to the existence of all kinds of negatively valenced states, and not
limit them to human-like cases of pain involving rich sensory representation.
3
See Walters (2018) for a recent review.
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Crook and Walters (2011), for instance, argue that gastropods show nociceptive
sensitization, which Godfrey-Smith (2020b) describes as “a heightened sensitivity after
damage” (p. 1155) and sees as compelling evidence for perhaps a minimal sense of
evaluative experience. What this work has shown is that when gastropods are exposed
to aversive stimuli such as electric shocks, they not only react with an immediate
behavioral response, but there also appears to be a long-term change in behavioral
“character.” Crook and Walters (2011) argue that Aplysia show a conditioned fear-
like motivational state when exposed to neutral chemosensory stimuli such as a touch
when it has in the past been associated with an electric shock (p. 189). Indeed, already
in 1981 Walters had shown that gastropods have the capacity for associative learning
(see Carew et al. 1981; Walters et al. 1981; Colwill et al. 1988), thus suggesting an
underappreciated level of evaluative richness that may be indicative of sentience.
When the smell of a shrimp was paired with an electric shock, Aplysia showed
surprising results in response to these stimuli in the future, such as (1) freezing in
response to the smell even in the absence of electric shocks, (2) halting of feeding
when exposed to the smell, and (3) withdrawal, escape, and defense responses when
the smell was paired with light touch (Crook and Walters 2011, p. 189).
From exchanges with Walters, Godfrey-Smith reports that Walters is more
cautious about attributing sentience to Aplysia, but that he also acknowledges the
striking functionalist rationale of an “ability to maintain functional ‘awareness’ of
injury-induced vulnerability until the vulnerability subsides (perhaps until adequate
repair of damaged body parts has been achieved)” (Walters 2018, p. 13; cited in
Godfrey-Smith 2020b, p. 1155). So, it is hardly surprising that Godfrey-Smith
(2020b) rightfully characterizes this surprising range of evaluative responses as
compelling evidence for a “pervasive state of negative readiness” linked to the
feelings side of subjective experience (p. 1155).
If the pathological complexity thesis is right, then this is exactly how the
vulnerability of complex multicellular organisms gives rise to hedonic experience.
One may even see these negative mood states as involving a minimal sense of self
and a sense of time. But these features need not be part and parcel of the subjective
experience of an animal in order to make particular stimuli associated with a negative
valence. After all, even humans can have a negative emotional reaction to an event
or food item without the ability to consciously draw the connection to a negative
encounter in the past. Nevertheless, it is tempting to think that episodic memory can
be readily explained as something built on these capacities once they are in place and
we should resist the thought that current boundary cases for the attribution of
sentience must be anything like the animals in the early evolution of subjective
experience. It is not at all implausible to think that the presence of a hedonic
evaluation system quickly gives rise to further increases in phenomenological
complexity.
Furthermore, Godfrey-Smith (2020b) praises Walters for highlighting the
distinctive life histories of Aplysia which often involve longer life cycles, of one to
two years more than is common in many insects. If we try to explicate the
pathological complexity of gastropods one will quickly find an additional rationale
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for these long-term mood states. Because their behavior is relatively limited in
comparison to many other animals that are discussed as potential bearers of
sentience, wound tending does not appear to be within their option space. Yet that
doesn’t mean that gastropods aren’t vulnerable. Unlike insects whose bodies are hard,
many slugs and snails lack even shells to protect themselves. But whereas insect
bodies can often not be “repaired,” hence making protection superfluous, gastropods
almost constitute a polar opposite case, with excellent if not extreme abilities to heal.
As long as wounds are not mortal, they will quickly restore their bodies to a healthy
state of normalcy again.
4
An extraordinary case in the genus Elysia cf. marginata
reported by Mitoh and Yusa (2021) has recently gained a lot of attention, since these
slugs have been shown to be able to decapitate their own heads from their body,
which includes shedding the entire heart, in order to rid themselves of a potentially
parasite-infested body. This is an extreme case of autotomy (i.e., the not-uncommon
behavioral strategy of deliberately shedding body parts or self-amputation), enabled
by the special regenerative modes of being of gastropods. This is one way of
responding to the pathological complexity of the gastropod life cycle. But it is also
precisely in this context in which behavior is limited, and bodies are vulnerable yet
allow for healing, that it makes sense to invest both in short-term states of pain and
in longer-term mood states such as fear or pessimism. Note, that I am here not
arguing for the thesis that they must be conscious, only that we have to think about
their life histories to even begin to think about what kinds of subjective experience it
would be worth having. Not too much should be made here out of the associations
with certain rich human emotions and mood states. What we are interested in are
these states as natural phenomena, which makes the human case a special case.
Due to the small nervous system that has made Aplysia a model organism to
begin with, these results provide compelling functional evidence that a minimal
degree of sentience may be present in these slow and vulnerable creatures. This view
isn’t anti-neural as much as it is gradualist. Because the genus Aplysia includes the
largest sea slugs, especially sea hares (Anaspidea), among which the California sea hare
(Aplysia californica) stands out in particular, which are comparatively much more
active—their movement resembling a “gallop rather than a slow crawl,” as Godfrey-
Smith notes—it can be hard not to grant them experience (2020a, p. 216). But despite
their behavioral difference from smaller sea slugs that have very similar nervous
systems yet lack this intuitive compellingness to be attributed sentience, this may have
merely been one of perspective, with Godfrey-Smith arguing that once they are scaled
up to the largest among the Aplysia, it becomes hard to draw a hard boundary of
experience; doubly so if their movement is sped up. A gradualist picture is tempting
here, and fits better with the actual data than the demand for a hard line (Veit and
Huebner 2020). Even tiny slugs and their ancestors may possess a basic capacity for
evaluative experience, despite a lack of capacities on the other dimensions. But let us
examine the sensory side of experience as well, in order to make sure that they do
only have evaluative experiences.
4
I am here employing a naturalistic sense of health (see Veit 2021).
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Sensory Experience
In the previous section I mentioned that gastropods seem to have fewer degrees of
freedom in their behavioral repertoires compared to insects. Furthermore, they have
much simpler sensory capacities, though there are some exceptions. Godfrey-Smith
(2020b), for instance, notes that sea elephants or heteropods (Pterotracheoidea) have
something of a borderline case of class IV eyes, i.e. high-resolution vision, which
might provide compelling evidence for sensory experience on the visual side. What
also distinguishes the lifestyles of these species is that they are much more mobile;
they have fins for free swimming and engage in predation, in contrast to most
gastropods that live on the ground. The pathological complexity they are faced with
is quite different from the usual sluggish gastropod way of life. For these swimming
gastropods, with lifestyles more closely resembling the pathological complexity of
fish and cuttlefish, we can make predictions regarding the likely richness in their
sensory experience. If sensory information processing (whether conscious or
nonconscious) is found in various degrees of complexity within a branch of life that
is already a likely contender for minimal sentience due to their rich evaluative
capacities, the pathological complexity thesis appears to gain striking support for the
close relationship between complex life histories and evaluative experience. Sensory
experience may be important for many evaluative capacities of consciousness, but it
does not appear to be necessary and should be seen as an enrichment that pays off
with higher degrees of pathological complexity that make sensory experience worth
having.
After all, as life histories become more complex, and thus involve greater
pathological complexity, the evaluative experience of organisms is bound to benefit
from greater discriminatory capacities to allow for the distinguishing of different
stimuli, their evaluation, and ultimately their comparison in order to optimize action
selection. Most gastropods, however, appear to only have a “sliver of the features
that make for experience in us” (Godfrey-Smith 2020a, p. 262), and this sliver appears
to be mostly on the evaluative side, thus providing compelling evidence for the
independent existence of evaluative experience without strongly representationalist
sensory capacities. Let us now turn to the case of insects which, if Godfrey-Smith is
right, may undermine the pathological complexity thesis due to their possession of
sensory without evaluative consciousness.
Arthropods: A Robotic Way of Life
Whereas Godfrey-Smith (2020b)’s arguments for the presence of evaluative
experience in gastropods without the sensory side provides strong support for the
pathological complexity thesis, his arguments for the existence of sensory experience
without the evaluative side in insects provides an interesting challenge to the idea that
the core of consciousness is to be found in evaluation, which we will ultimately have
to overcome. Godfrey-Smith (2020b) suggests that complexity in sensory “capacities
might be understood as involving complexity in discrimination or in downstream
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processing” (p. 1153), but emphasizes the latter as being more important for
considerations of subjective experience. That is certainly reasonable due to a
recognition of how many discrimination activities are going on without subjective
experience even in humans, but Birch et al.’s (2020) dimensions of animal
consciousness characterize evaluative richness as complexity in discrimination. For
the purposes of the discussion here, I agree with Godfrey-Smith’s emphasis on
downstream processing, since the pathological complexity thesis sees sensory
experience as something operating within an evaluative sphere. It is only in this
hedonic context that these sensory processes are felt, rather than any and all cognitive
processes involving discrimination.
5
Let us therefore now closely examine the
challenge that sensory experience could exist without such an evaluative space in
which different sensory stimuli are being evaluated against each other.
Insects are part of the arthropod branch of life and constitute the great
majority of arthropod species (in addition to all animals!), but are estimated to have
originated only roughly 479 million years ago during the early Ordovician. This, as
Misof et al. (2014) point out in a landmark study in Science, suggests that they have
evolved in response to the plants that started to colonize the planet around the same
time (see also Labandeira 2006). However, the arthropod group which also includes
crustaceans (e.g., crabs, lobsters, and krill), arachnids (e.g., spiders and ticks), and
myriapods (e.g., centipedes), are a much earlier Cambrian invention; indeed they
constitute the paradigm phylum of the Cambrian explosion, leading the way for a
special animal way of life. Their name, a conjunction of the Ancient Greek words for
“joint” and “foot,” is a fitting description for a mobile mode of being consisting of
hard shells, multiple segments, and typically many appendages (Budd and Telford
2009), that nevertheless shares a common active lifestyle with the “soft” and
“sluggish” gastropods. But despite sharing a high degree of pathological complexity,
it plays out differently in both groups, and this might make it tempting to think that
arthropods could evolve sensory consciousness without the presence of evaluation.
To examine this further, we will this time begin with the sensory side of
consciousness.
Sensory Experience
Unlike the soft-bodied gastropods, arthropods seemingly overflowed in the
Cambrian, with trilobites making up much of the fossil record. Partially this is due to
their possession of an exoskeleton, which simply fossilizes better, but their presence
emphasizes much of the change that took place during the Cambrian. An exoskeleton
makes sense as a protective shell against others, with appendages such as feelers and
claws clearly existing in response to other subjects, whether prey, partner, or predator.
Godfrey-Smith (2020a) describes arthropods as having “invented a new way of being
an animal, with a skeleton that scaffolds and organizes complex actions. They also
invented claws, and to go with them, image-forming eyes” (p. 80). All this appears
5
A related problem is the misidentification with cognitive complexity, or “intelligence” with
sentience (Browning 2019b).
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much richer than the sensory capacities and behavioral repertoire of gastropods and
it is not surprising that Feinberg and Mallatt (2016) in their evolutionary overview
also emphasize this richness. These facts may actually make it surprising that few
have granted them a minimal sense of subjective experience despite the vision-centric
model of consciousness that is prevalent, since many insects have been shown to
have sophisticated sensory capacities and especially high-resolution vision. Again, I
want to note that I am here not arguing for the thesis that arthropods necessarily
have consciousness, only that if they do they are likely to have sensory as well as
evaluative experience, rather than just the sensory side.
6
Godfrey-Smith (2020b) focuses especially on the much-studied bees and fruit
flies (Drosophila), since it is here that we can examine flight as a complex behavior that
“involves dealing with complex spatial layouts and making self/other distinctions
with respect to the causes of sensory events” (p. 1153). Indeed, in the ecological
framework for the comparative study of consciousness that I try to build, flight
constitutes the paradigm case for an explosion in pathological complexity. Godfrey-
Smith doesn’t commit himself here but sees the sensory processing of flying insects
as a plausible candidate for subjective experience. That flying creates a new challenge
of complexity is not a new idea.
In his very first publication, the British evolutionary biologist and former
aeronautical engineer John Maynard Smith (1952) already emphasized the importance
of a sophisticated nervous system in the evolution of flight, for both birds and insects.
He argued that the evolutionary origins of flight must have required flight stability
via a long tail, since they lacked the sensory richness and nervous system complexity
to control such a flying body, similar to how pilots require a stable plane in order to
be able to fly it. While such a tail lowers maneuverability, it greatly increases flight
stability. Yet Maynard-Smith argued that “in the birds and at least some insects, and
probably in the later pterosaurs, the evolution of the sensory and nervous systems
rendered the stability found in earlier forms no longer necessary” (1952, p. 129). The
evolutionary advantages of unstable flight, he argued, would be the ability to turn
more rapidly in the air and to be able to fly at slower speeds without falling (1952, p.
128). Taking a design stance toward the problem of flight makes it obvious how rich
the complexity of this problem really is. Free fall can mean death. But Maynard Smith
made these comments in relation to birds. Because insects are so small, air resistance
will stop them from gaining enough fall speed to cause serious injury. Nevertheless,
it is precisely because of their size that it is more important to focus on the
organization of the insect nervous system, rather than the mere number of neurons.
Regardless of the possible challenges that can be raised to the presence of sensory
consciousness in arthropods and/or flying insects, however, it (1) makes sense within
the pathological complexity framework and (2) will be accepted in order to tackle
6
While the pathological complexity thesis certainly increases the credence for the
presence of sentience in insects, their very small nervous systems may provide a compelling
reason to think that at least some branches of insect life might have lost consciousness. The
minimal nervous system requirements for sentience will be a matter for another paper.
11
Godfrey-Smith’s challenge that the sensory side could exist without the evaluative
side.
Evaluative Experience
In order to make his case for a potential separation of the evaluative and sensory
sides of experience in insects, Godfrey-Smith draws on an old but influential mini-
review by Eisemann et al. (1984) in order to establish that “all known insects appear
completely unconcerned about even severe body damage. Wound-tending has never
been seen in an insect, and after injury these animals just continue, as best they can,
with the behavior appropriate to the circumstances” (Godfrey-Smith 2020b, pp.
1153-1154). But this is partially a misrepresentation of even this early work on the
possibility of insect pain. Indeed, Eisemann et al. (1984) cite early experimental work
by the German zoologist Erwin Hentschel (known in Germany as the “bee
professor”
7
) that showed grooming activity in response to damage (Hentschel et al.
1982) as something to be explained. They explicitly recognize an “increase in both
general grooming activity and specific grooming of a wound site observed after
experimental puncturing of the abdominal wall of the cockroach Periplaneta americana
(L.)” (1982, p. 166).
While it is true that Eisemann et al. (1984) argue that insects do not feel pain,
they do so in a very measured way, only stating that “the evidence from consideration
of the adaptive role of pain, the neural organisation of insects and observations of
their behavior does not appear to support the occurrence in insects of a pain state,
such as occurs in humans” (p. 167). That they see the evidence as far from conclusive
is also emphasized by their call to endorse Wigglesworth’s earlier recommendation:
[…] that insects have their nervous systems inactivated prior to traumatizing
manipulation. This procedure not only facilitates handling, but also guards against the
remaining possibility of pain infliction and, equally important, helps to preserve in the
experimenter an appropriately respectful attitude towards living organisms whose
physiology, though different, and perhaps simpler than our own, is as yet far from
completely understood. (Eisemann et al. 1984, p. 167)
8
This makes it somewhat puzzling as to why Godfrey-Smith (2020a) similarly repeats
his assertion in Metazoa that “insects have still never been observed tending and
grooming injuries; that claim from the old no-pain paper still holds up” (pp. 211-
212).
9
Just because insects have not been shown to engage in sophisticated
“protective behavior towards injured body parts, such as by limping after leg injury
or declining to feed or mate because of general abdominal injuries” (Eisemann et al.
7
https://idw-online.de/de/news15923
8
See also Wigglesworth (1980).
9
In personal communication (14 Aug 2021) Godfrey-Smith admitted that he should not
have used the term “grooming” in his list and aims to have it removed in a second editon of
his 2020b book Metazoa (see also Veit 2022e for a review of his book).
12
1984, p. 166) does not mean that no grooming-like behavior has been observed—
even if it could be explained in a way unrelated to pain. The way Eisemann et al. (1984)
deal with Hentschel’s observations is to point out the “contra-adaptiveness of this
response in relation to wound healing” (p. 166). But we have to distinguish the
adaptive value of such behavior from its support for the presence of subjective
experience. It may very well be the case that not all grooming behavior is adaptive,
no less so than itching or scratching of human wounds is. Pain could be invoked as
a cause as long as a general negative valence exists regarding damage or potential
damage. Indeed, this might even be seen as supporting the presence of negative
valence as opposed to a mere “mechanical” response.
My argument here, however, should not be read as me endorsing the presence
of pain in insects. I only argue that the case is not as straightforward in insects as
Godfrey-Smith makes it seem. Nevertheless, it is certainly true that insects—more so
than perhaps any other complex agent-like animal group—have been observed to be
apparently oblivious to all kinds of damages and injuries and such findings as well as
anecdotes certainly constitute important evidence (Browning 2017). They engage in
sex and eating while being devoured, soldier on despite damages, and even eat their
own insides when they are leaving behind their body due to damage (Eisemann et al.
1984; Adamo 2016; Walters 2018). There appears to be a striking failure to recognize
biological normativity in insect life. In order to understand whether such behavior is
functional or not, we will have to understand the pathological complexity faced by
insects. Godfrey-Smith (2020b) notes the “ecology of insects is also relevant” (p.
1154), but for a true cognitive ethology it should be our primary source of
information. Godfrey-Smith’s (2020b) references to the life history of insects versus
crustaceans is particularly interesting here in the context of the pathological
complexity thesis.
Whereas most crustaceans live in the water, having similar life history strategies
to their Cambrian ancestors, the insects have predominantly branched towards a life
on land.
10
Yet whereas Godfrey-Smith wants to deny evaluative experience in insects,
he grants it to crustaceans, where wound tending has been firmly established. The
work of Elwood and his collaborators (Appel and Elwood 2009; Elwood et al. 2012)
has studied the evaluative trade-offs crustaceans are engaged in, particularly decapod
crustaceans (shrimps, crabs, and the like). Hermit crabs have shown, for instance,
that they are making state-based decisions on whether or not to leave their shell when
receiving electric shocks, dependent upon both the predicted presence of predators
and the shell value. Furthermore, in a recent extensive review that has led to decapod
crustaceans being included in the animal sentience legislation in the UK, a wealth of
data has been presented to support sentience in these creatures (Birch et al. 2021; see
also Crump et al. 2022).
But this admittedly transforms the challenge to the pathological complexity
thesis. Instead of sensory experience arising independently in its own right, the
10
Crustaceans are likely a paraphyletic group (Blackstone 2001).
13
challenge now appears to be explaining a loss of the evaluative side once the sensory
side has come to play a more important role. After all, these results have motivated
Tye (2016) to call his book Tense Bees and Shell-Shocked Crabs: Are Animals Conscious? to
begin with. But Tye has of course neglected the importance of the underlying state-
based behavioral and life history theory in favor of his much more simplistic
epistemological “defeater” approach. Allen and Bekoff criticized as anti-neural
Griffin’s suggestion that bees might have more of a use for subjective experience
because their nervous systems are so small (1997, p. 153), but it is based on the
important insights of Lorenz and Tinbergen for putting a firm understanding of the
animal’s life history prior to laboratory experiments.
Within the peculiar pressures of life on land, most insects have evolved short
and routinized lives that differ from the comparatively longer and “less regimented
lives of their marine relatives studied by Elwood” (Godfrey-Smith 2020b, p. 1154).
While there are exceptions to this rule (Maruzzo and Bortolin 2013; Suzuki et al.
2019), insect limbs generally do not regrow and there is little evidence that there is
adaptive value for them in protecting injuries. Godfrey-Smith (2020b) describes this
lifestyle as being about soldiering on even in the face of pathologies (p. 1154). Now,
this makes sense in a semelparous life and especially so in eusocial insects where
individuals can be replaced. One might expect bees or ants to have sophisticated
sensory capacities for finding food sources but being less rich on the sensory side in
order to focus on their task. But does this really show that the evaluative side has been
lost?
Godfrey-Smith (2020b) admits that bees have been shown to avoid noxious
stimuli such as heat, but notes that this could be a mere reflex, not necessarily
involving subjective experience. A compelling line of evidence in this context is
various kinds of learning, since they are commonly taken to increase our confidence
in attributing evaluative experience. Also highlighted by Godfrey-Smith is
reinforcement learning which has been shown in various insects (see Allen et al. 2005;
Elwood et al. 2012) and is often drawn on in discussions of consciousness. Which
forms of learning constitute the best kinds of evidence is, however, contested.
11
But
as with gastropods, we should also highlight evidence for nociceptive sensitization as
indicative of evaluative richness in insects, which is also noted by Tye (2016). One
peculiar result that Godfrey-Smith (2020b) highlights, is the presence of sensitization
in Drosophila larvae, as opposed to its later life stages (p. 1156). Too much focus, he
11
Ginsburg and Jablonka (2019) provide a good overview of this debate and argue that
there is a form of learning that they dub “unlimited associative learning” (UAL) that provides
something like a proof that animals are conscious; though they do not mean to say that the
absence of UAL shows that consciousness isn’t present. I am very skeptical that we can
actually find anything like a definite marker, since consciousness can come in a diversity of
forms, but that is not an objection to the idea that sophisticated associative learning would
constitute a good indicator for a certain richness or even transition of consciousness, as
opposed to its presence (see Browning and Veit 2021 for a review of their work).
14
notes, might have been given by Eisemann et al. (1984) and Groening et al. (2017) to
the absence of pain in adults:
Another factor in insects not highlighted so far, one related to life on land, is
the differences between larval and adult states. Many insects lead two lives, in
effect, one on each side of a metamorphic divide, with extensive breakdown
and reconstruction at that stage. In the kinds of insects considered here, it is
the adult who has acute sensing that controls complex motion; the larva does
not. (Godfrey-Smith 2020b, p. 1156)
Drawing on Sprecher et al. (2011), Godfrey-Smith emphasizes that larvae have very
simple eyes—in Drosophila only the small number of 12 photoreceptor neurons
dedicated for vision, much simpler than the adult stages. Yet, in contrast to the
apparent obliviousness to damage in adult insects, Godfrey-Smith (2020b) also
highlights work by Walters (2018) that showed larval stages of Manduca and Drosophila
to have nociceptors and nociceptive sensitization. What we find in insects is a striking
disconnect between the pathological complexity faced by the larval and the adult
stages. As Walters (2018) observes: “Trade-offs between survival and reproductive
success are found in all animal groups but seem especially striking in insects” (p. 12).
In part, such observations are due to the extreme diversity of insect life, which
explains the presence of a huge variety of alternative behavioral life history strategies;
including such odd examples as male mantids engaging in sex with females despite
being eaten afterwards. While this behavior may well be adaptive (Schwartz et al.
2016; Zuk 2016), it is hard to think about such extreme behaviors involving pain.
And yet, larvae—despite their nervous system simplicity—often appear to have
richer evaluative capacities than adults, indicative of the different emphasis of
survival during this stage, as opposed to reproduction in the adult one. The adult
insect body is described by Godfrey-Smith (2020b) as a mere tool for this end. So, it
would make sense to have life-stage-dependent varieties of experience. This is
something that can straightforwardly be captured within the pathological complexity
framework, providing us with a measure for the different stages of a life history
(though I will not go into the mathematics here). Indeed, we can use this framework
to integrate work on the mechanism, development, function, and evolutionary history
of consciousness.
A recent compelling case for insect consciousness has been made by Barron
and Klein (2016), yet they largely emphasize the sensory side of things. Findings of
the evaluative side are more compelling than they are typically given credit for.
Godfrey-Smith (2020b) points to self-administration of analgesics which has been
used as compelling evidence of pain in birds and fish, yet has not been found in bees
(Groening et al. 2017), but that is not the only source of evidence we can look for.
Bateson et al. (2011) show convincingly that bumblebees, if they have been shaken,
can have negative long-term mood states called pessimistic bias. Similarly, Godfrey-
Smith (2020b) admits that bees and other insects show aversive responses to heat,
which may be better stimuli to look for the presence of subjective experience.
15
As I have argued elsewhere, even if insects do not experience human-like pain
towards mechanical injuries, they may very well experience other aversive experiences
such as hunger or thirst (Browning and Veit 2020a). The absence of pain is too often
confused with the absence of evaluation. But the lifestyles of insects simply don’t
make it necessary to put much of a value on protecting one’s bodily shell from
mechanical damage. What is important is to complete one’s life history strategy: i.e.,
to reproduce. If wound tending doesn’t aid that, there is little sense in putting much
valence on it. Instead of focusing on pain-like behavior as an admittedly tempting
but flawed paradigm case of evaluative experience, we should look at evidence for a
valence system more generally that evaluates trade-offs between conflicting stimuli
in a flexible manner. Evidence that is very compelling here, and highlighted by
Godfrey-Smith (2020b), is a follow-up study to Bateson et al. (2011), which focused
on positive mood states in the form of optimism bias in response to unexpected
rewards in bees (see Perry et al. 2016). The idea of pleasure as a common currency
for affective decision-making is sometimes criticized as failing to account for the
different neural mechanisms of negative and positive evaluation, but such common
functional roles of evaluation suggest that they are deeply evolutionarily intertwined.
Indeed, they must largely operate in tandem to allow for efficient decision-making
and learning in the face of novel and ambiguous stimuli.
12
Given how much we know about sensory processing taking place
unconsciously in human brains, it appears plausible to think that it is only those
sensory inputs that enter the affect system of the brain that are consciously
experienced (see also Ginsburg and Jablonka 2019). It provides us with an answer to
those who insist that functionalist accounts of consciousness cannot explain the
“feel” of experience, since it is precisely this subjective experiencing that does the
functional work. It enables organisms to efficiently deal with their species-specific
pathological complexity. So while we can readily admit that insects do not feel pain
due to their “robotic” way of life, their complex behaviors and learning abilities are
highly suggestive of something like a common currency of valence for efficient action
selection, even if their evaluative capacities on this side of things may have become
poorer compared to their sea-living ancestors. Insects, after all, are in many ways the
scaled down versions of their Cambrian ancestors, with a constant pressure for
nervous system simplicity, especially in those insects that can fly. It is because of this
that they might provide us with an insight into the minimal nervous system
requirements for sentience. Lack of evidence should here not be confused with
evidence of absence precisely because dedicated research on their evaluative
capacities (as opposed to their sensory capacities) has been rare. But this is now
beginning to change.
Indeed, on the 8th of February this year, a particularly compelling preprint on
motivational trade-offs in bumblebees was uploaded by Lars Chittka’s bee lab (see
Gibbons et al. 2022). In it, the authors showed that bees when faced with noxiously
12
While metaphors can sometimes impede scientific progress (Veit and Ney 2021), the
metaphor of a common currency is highly useful to think about the evolution of sentience.
16
heated feeders and different sugar concentrations could trade off “competing
conditioned motivational stimuli to modulate nocifensive behaviour” which they
argued would suggest “a form of pain perception” (Gibbons et al. 2022, p. 1). This
suggests that even the presently best case for the independent existence of sensory
experience without evaluative experience turns out to have rich evaluative capacities
after all, supporting the motivation of the pathological complexity thesis to seek the
origins and core of subjective experience in hedonic valence without the need for
other dimensions of consciousness to necessarily be present (see also Veit 2022f).
Arthropods may have a robot way of life, but that doesn’t mean that they do not
engage in complex evaluations.
Conclusion and Further Directions
In this article, I have discussed numerous animals such as snails, slugs, fruit flies,
crabs, and bees, that many if not most will suspect not to have sufficient nervous
system complexity to warrant an attribution of consciousness. While the scientific
assessment of consciousness in nonhuman animals has methodological roadblocks
(Browning and Veit 2020b), I hope to have made clear in this article that it is precisely
in such comparatively “simple” animals that we have to begin an evolutionary
investigation of consciousness. They are the best cues to what the gradual evolution
of subjective experience may have been like, providing us with a rich diversity of
alternative life history strategies. If these animals are conscious, their consciousness
profiles would appear to be restricted to the dimensions of evaluative experience and
sensory experience, with perhaps some sophistication on the selfhood side with an
implicit distinction between exteroperception and interoperception (though this
could also be seen as richness of the sensory side of things). My discussion here was
framed against recent work by Godfrey-Smith (2020a, b) who emphasized the
possibility of a disassociation between the sensory and evaluative side in gastropods
and insects, which may yield us a two-by-two table representing capacities across each
dimension.
Since the pathological complexity thesis seeks to locate the origins of
consciousness in evaluation, Godfrey-Smith’s analysis of gastropods provides
excellent support for the idea that there have been organisms at the beginning of the
Cambrian with evaluative capacities sufficient for sentience, but lacking sensory
experience. Indeed, such organisms could be around even now, undermining the
common idea that consciousness must necessarily involve a great cluster of capacities
related to consciousness in humans. Godfrey-Smith’s case for the independent
existence for sensory experience, however, has constituted an interesting challenge
to the pathological complexity thesis, since my framework explained the phenomenal
feel of sensory experience as a discriminatory capacity of a more basic evaluative
capacity for experience.
As I hope to have shown in this article, however, this second conceivable
separation must not constitute a real separation in nature. Even insects with their
apparent disregard for their own bodies seem to have rich evaluative capacities,
17
making the attribution of exclusive sensory experience without evaluation a less
promising contender. Whereas most discussions of consciousness focus on animals
that plausibly have together at least a minimal sense of both sensory and evaluative
experience, such as most vertebrates and octopuses, a comparative bottom-up
approach allows us to reverse engineer the origins of consciousness by emphasizing
the animals in which consciousness plausibly exists in only a rudimentary form. While
I have agreed with Godfrey-Smith in his assessment of the exclusive presence of
evaluative experience in gastropods, I have argued against his suggestion of the
exclusive presence of sensory experience in insects. Evaluation, as the pathological
complexity thesis maintains, may indeed be at the core of consciousness—though we
should also recognize that the animals existing in the here and now are not necessarily
representative of the earliest sentient beings that may have had even more
rudimentary forms of hedonic evaluation than gastropods that have had millions of
years to fine-tune this capacity.
Finally, the arguments here should importantly not be misunderstood as
definite proofs. My goal was not to make anything like a final assessment of the
subjective experience of arthropods and gastropods, but rather to revive the
comparative spirit of Griffin’s cognitive ethology, in order to use functionalist
reasoning about what kinds of subjective experience would make sense given the
different life history strategies of different animals. The motivation of the
pathological complexity thesis to seek the origins of consciousness in evaluation may
well turn out to be ultimately misguided. But if we want to develop a true biological
science of consciousness, we must begin with a firm understanding of the healthy
lifestyles of different species in the normal natural environments they have evolved
in. Only then can we answer the functionalist question of what consciousness is for,
which can in turn help us to answer the question of what kinds of conscious
experiences would be worth having. In asking for the possibility of sensory
consciousness, or rather the extent of the subjective worlds of animals, we should
ask which kinds of felt discriminations of distinct stimuli are useful for the kinds of
evaluative activities a species is engaged in during its life cycle. The pathological
complexity thesis offers us a framework to at least make some progress on these
difficult problems by using the Darwinian lens of state-based behavioral and life
history theory, which could, for instance, lend itself towards making predictions
about color vision in species such as bees that are routinely engaged in economic
trade-off calculations in their foraging activities. Within the scope of this article, I
have restricted myself to fairly general thinking about sensory and evaluative
experience in gastropods and arthropods, but future work will inevitably take a much
more narrow and precise approach focusing on particular species in order to make
predictions that could in turn be tested and corroborate the framework presented
here.
Unfortunately, the importance of evolutionary thinking has so far been
neglected in a science that was hard-pressed by the behaviorist Zeitgeist to make itself
seem as objective as possible (Birch et al. 2022). But the time has come to draw on
the greatest strength a Darwinian approach to biological phenomena has to offer: the
18
comparative method. If nothing else, the pathological complexity thesis provides an
evolutionary framework for just such a bottom-up comparative study of animal
consciousness that might also enable us to solve the problem of interspecies
comparisons of subjective welfare by providing something like a “sentience-
multiplier” (see Browning 2022a, b, c). I hope that this article has shown that a
biologically well-informed understanding of the diverse life history strategies we find
in nature can and should play an important role in discussions of animal
consciousness. Overcoming the resistance to such an approach may allow us to—in
Griffin’s words—write the final, crowning chapter of the Darwinian revolution.
Acknowledgments
My thanks go out to Peter Godfrey-Smith, Paul Griffiths, Marian Dawkins, and Heather
Browning for their feedback on the pathological complexity thesis. I would also like to thank
members of Jonathan Birch’s Animal Sentience lab at the London School of Economics and
Nicky Clayton’s comparative cognition lab at the University of Cambridge for helpful
discussions. Lastly, I would like to thank Stuart Newman, Deborah Klosky, and reviewers at
Biological Theory for helping me to polish the manuscript further.
Competing Interests
None.
Funding Information
WV’s research was supported under Australian Research Council’s Discovery Projects
funding scheme (project number FL170100160).
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